1. Field of the Invention
Example embodiments of the present invention relate to an image forming apparatus, a fixing apparatus, a toner, and a method of preparing a toner.
2. Description of the Background Art
Methods for forming an image by developing an electrostatic latent image in an image forming apparatus, for example, electrophotographic and electrostatic recording methods, are now used in various fields. In the electrophotographic method, an electrostatic latent image is formed on a photoconductor based on image data through charging and exposing processes. The electrostatic latent image is developed with a developer into a toner image and transferred onto a recording medium, for example, paper, through developing and transfer processes. The toner image is fixed through a fixing process so that an image is formed on the recording medium.
Background image forming apparatuses, for example, copiers, facsimiles, and printers, generally include a fixing unit for fixing the toner image transferred on the recording medium.
One example of a fixing unit includes a pair of rollers, e.g., a heating roller and a pressure roller opposing to each other. A recording medium having a toner image is conveyed through a nip formed between the heating roller and the pressure roller. Heat applied by the heating roller and pressure applied by the pressure roller melt and fix the toner image on the recording medium.
Another example of a fixing unit includes the pressure roller, a fixing belt replacing the heating roller of the above example, and a pair of rollers for rotating the fixing belt. The fixing belt is looped over the pair of rollers. One of the rollers opposes the pressure roller via the fixing belt. The other roller includes a heater for heating the fixing belt from its inner circumferential surface and the pressure roller includes another heater for heating the fixing belt from its outer circumferential surface. The fixing belt can be heated more quickly than the heating roller of the above example due to its smaller volume and heat capacity. Thus, this fixing unit can be heated to a desired temperature more quickly than the above fixing unit including the heating roller after the image forming apparatus is powered on. The two heaters respectively heat the inner and outer circumferential surfaces of the fixing belt, resulting in quick increase in temperature of the fixing belt.
Yet another example of the fixing unit includes a fixing roller, a heating roller, a fixing belt formed in an endless belt shape and looped over the fixing roller and the heating roller, and a pressure roller opposing the fixing roller via the fixing belt. A heater for heating the fixing belt is placed inside any one or each of the pressure roller and the heating roller. A recording medium having a toner image is conveyed between the fixing belt and the pressure roller. The toner image on the recording medium is fixed while the recording medium passes a first fixing area where the pressure roller applies pressure to the fixing belt via the recording medium and does not apply pressure to the fixing roller via the recording medium and the fixing belt. In the first fixing area, the pressure roller applies a low pressure not creasing the recording medium. The toner image on the recording medium is also fixed while the recording medium passes a second fixing area where the pressure roller applies pressure to the fixing roller via the recording medium and the fixing belt. In the second fixing area, the pressure roller applies a level of pressure enabling a desired fixing. Thus, the recording medium is properly conveyed and the toner image is stably fixed on the recording medium even in a high speed or color image forming apparatus.
When the toner image is fixed by using the fixing belt and the pressure roller, the outer circumferential surface of the fixing belt may be charged and may attract toner particles from the recording medium. This is called an electrostatic offset.
When electrostatic offset occurs, the attracted toner particles may be transferred onto another recording medium after the fixing belt rotates for one cycle and may form an afterimage on the recording medium. To reduce or prevent this, a cleaning member (e.g., a cleaning roller) is disposed to contact the fixing belt to remove the attracted toner particles from the fixing belt. However, when a large amount of toner particles is adhered to the cleaning member, the toner particles may melt onto the fixing belt, resulting in staining and damaging the recording medium.
Image forming apparatuses using the electrophotographic and electrostatic recording methods should produce images having an improved transparency and saturation.
The developer used in the developing process in the electrophotographic method includes a one-component developer containing a magnetic toner or a non-magnetic toner and a two-component developer containing a toner and carriers.
A toner used as the developer is generally produced in a mixing-kneading-pulverizing method in which a thermoplastic resin and a pigment are dissolved, mixed, kneaded with a releasing agent for example, wax and/or a charging control agent, if necessary, and then pulverized and sized. To improve fluidity and cleaning property of the toner, inorganic or organic fine particles are added to surfaces of toner particles, if necessary.
The toner particles produced in the mixing-kneading-pulverizing method generally have no definite shape, and have a broad particle size distribution, a low fluidity and transferability, a high fixing energy, a charging amount varying depending on toner particles, and a low charging stability. An image formed with such toner particles may provide insufficient image quality.
A polymerization method is proposed to solve the above problems of the toner particles produced in the mixing-kneading-pulverizing method. The polymerization method does not include kneading and pulverizing processes, resulting in cost reduction caused by energy saving, shortened production hours, and an improved yield of products. A sharper particle size distribution can be easily obtained with toner particles produced in the polymerization method than with the toner particles produced in the mixing-kneading-pulverizing method. In the polymerization method, wax can be contained inside the toner particles to improve fluidity of the toner particles and the toner particles can be formed in a spherical shape.
However, the toner particles produced in the polymerization method have problems. A surface tension affecting the toner particles during a polymerization process produces toner particles having a sphericity higher than that of the toner particles produced in the mixing-kneading-pulverizing method. However, physical properties of the toner particles produced in the polymerization method are not sufficient. In the polymerization method, it is not easy to control (e.g., vary) a shape of the toner particles. However, the polymerization method can have an advantage in producing toner particles having an improved charging stability and transferability.
The polymerization method includes a suspension polymerization method generally used. Monomers for a binder (e.g., a binder resin) used in the suspension polymerization method may be limited to a styrene monomer and an acrylic monomer harmful to humans. Toner particles produced in the suspension polymerization method contain those monomers and may cause environmental problems. Since wax is contained inside the toner particles, a decreased amount of the toner particles are adhered to a photoconductor. However, the toner particles produced in the suspension polymerization method have a lower fixing performance than the toner particles produced in the mixing-kneading-pulverizing method. In the mixing-kneading-pulverizing method, wax is on an interface of the toner particle. In the suspension polymerization method, the wax contained inside the toner particles does not easily seep onto surfaces of the toner particles, resulting in a low fixing performance. Therefore, the toner particles produced in the suspension polymerization method (e.g., polymer toner particles) may have a disadvantage in reducing energy consumption. If an amount of the wax or a dispersed particle size of the wax is increased to improve the fixing performance of the polymer toner particles, transparency of a color image may deteriorate when the polymer toner particles are used for forming the color image. Thus, the polymer toner particles are not suitable for forming a color image on an OHP (overhead projector) transparency used for presentation.
The polymerization method further includes an emulsion polymerization method which can vary the shape of toner particles. A monomer used in the emulsion polymerization method is limited to the styrene monomer. It may be difficult to completely remove an unreacted monomer, an emulsifier, and a dispersing agent from the toner particles, causing environmental problems.
A dissolution-suspension method is also known as a toner production method. The dissolution-suspension method may have an advantage in using a polyester resin enabling fixing at a low temperature. A high-molecular-weight component is added in a process of dissolving or dispersing a resin enabling fixing at a low temperature and a colorant in a solvent. Therefore, a liquid viscosity may increase, causing problems relating to production performance. Toner particles produced in the dissolution-suspension method are formed to have a spherical shape and a patterned indented surface in order to improve cleaning performance for the toner particles. However, those toner particles having an amorphous shape without regularity may lack charging stability and may have problems in endurance and releasing, providing insufficient quality.
A dry toner particle is proposed to improve fluidity, fixability at a low temperature, and hot offset resistance. The dry toner particle includes an elongated reactant of urethane-modified polyester as a toner binder and has a practical sphericity ranging from 0.90 to 1.00. Another dry toner particle proposed may have an advantage in powder fluidity and transferability when formed as a toner particle having a small particle size as well as in heat-resistant preservation, fixability at a low temperature, and hot offset resistance. Methods for producing the above dry toner particles include a high-molecular-weight producing process of polyadding polyester prepolymer having an isocyanate group with amine in an aqueous medium.
In the polymer toner produced in any one of the above polymerization methods, however, a pigment is not properly dispersed but is unevenly dispersed in the toner. Thus, an image formed with the toner may have an inferior transparency and saturation (e.g., brightness). Particularly, when a color image is formed on an OHP transparency with the toner, the color image may become dark.